This invention relates to an internal combustion engine, and more particularly to lubrication and cooling of the cap side of the larger end portion of a connecting rod, i.e., the end portion connected to the crankshaft, in engines of the type which do not employ a pressurized lubricating system, such as in a small two-stroke outboard engine.
As the r.p.m. of an internal combustion engine is increased, there is observed a phenomenon that the temperature at the cap side of the larger end portion of the connecting rod becomes remarkably higher than that at the rod side. As a result, separation takes place between the bearing and the crankshaft on the cap side. That separation exerts influences to other members, thereby to raise one of the causes for shortening the lifetime of the internal combustion engine. In order to cope with this phenomenon, therefore, the cap is formed with radial lubricating holes through which lubricant from the crankcase is directed between the bearing and the crankshaft, thereby to improve lubrication and cooling. That lubricating hole is conically countersunk, as if to receive a screw, but even such an enlarged inlet to the lubrication hole can hardly trap the lubricant to a sufficient extent as the r.p.m. of the engine is increased. As a consequence, the temperature rise at the cap side cannot always be restrained.